Patrons, and maybe the fish, at the New England Aquarium got quite a shock on Thursday when a man jumped into the large center tank that features marine animals including a shark, prosecutors said. Stephen Pellegrine, 51, of Quincy leaped into the tank and began swimming at 6:30 p.m., said Jake Wark, spokesman for the Suffolk district attorney. Aquarium staff demanded that he get out of the water, but he swam around for “several minutes” before exiting, Wark said. Pellegrine had a not-guilty plea entered on his behalf Friday in Boston Municipal Court to charges of trespassing and disorderly conduct, Wark said.